Phân biệt mạng trong Internet

Chia sẻ: Nguyen Van Thanh | Ngày: | Loại File: DOC | Số trang:12

0
166
lượt xem
33
download

Phân biệt mạng trong Internet

Mô tả tài liệu
  Download Vui lòng tải xuống để xem tài liệu đầy đủ

Wi-Fi (pronounced /waɪfaɪ/) is a trademark of the Wi-Fi Alliance for certified products based on the IEEE 802.11 standards. This certification warrants interoperability between different wireless devices.

Chủ đề:
Lưu

Nội dung Text: Phân biệt mạng trong Internet

  1. Wi-Fi (pronounced /ˈwaɪfaɪ/) is a trademark of the Wi-Fi Alliance for certified products based on the IEEE 802.11 standards. This certification warrants interoperability between different wireless devices. In some countries (and in this article) the term Wi-Fi[1][2] is often used by the public as a synonym for IEEE 802.11-wireless LAN (WLAN). Not every IEEE 802.11 compliant device is certified by the Wi-Fi Alliance, which may be because of certification costs that must be paid for each certified device type. The lack of the Wi-Fi logo does not imply that a device is incompatible to certified Wi-Fi-devices. Wi-Fi is used by most personal computer operating systems, many video game consoles, laptops, smartphones, printers, and other peripherals. Half-size ISA 2.4 GHz WaveLAN card by AT&T Wi-Fi uses both single carrier direct-sequence spread spectrum radio technology (part of the larger family of spread spectrum systems) and multi-carrier OFDM (Orthogonal Frequency Division Multiplexing) radio technology. The regulations for unlicensed spread spectrum enabled the development of Wi-Fi, its onetime competitor HomeRF, Bluetooth, and many other products such as some types of cordless telephones. Unlicensed spread spectrum was first made available in the US by the FCC in rules adopted on May 9, 1985[3] and these FCC regulations were later copied with some changes in many other countries enabling use of this technology in all major countries. The FCC action was proposed by Michael Marcus of the FCC staff in 1980 and the subsequent regulatory action took 5 more years. It was part of a broader proposal to allow civil use of spread spectrum technology and was opposed at the time by mainstream equipment manufacturers and many radio system operators.[4] The precursor to Wi-Fi was invented in 1991 by NCR Corporation/AT&T (later Lucent Technologies & Agere Systems) in Nieuwegein, the Netherlands. It was initially intended for cashier systems; the first wireless products were brought on the market under the name WaveLAN with speeds of 1 Mbit/s to 2 Mbit/s. Vic Hayes, who held the chair of IEEE 802.11 for 10 years and has been named the "father of Wi-Fi," was involved in designing standards such as IEEE 802.11b, and 802.11a.
  2. The original patents behind 802.11 Wi-Fi technology, filed in 1996, are held by the CSIRO, an Australian research body. The patents have been the subject of protracted and ongoing legal battles between the CSIRO and major IT corporations over the non- payment of royalties. In 2009 the CSIRO reached a settlement with 14 companies, including Hewlett-Packard, Intel, Dell, Toshiba, ASUS, Microsoft and Nintendo, on the condition that the CSIRO did not broadcast the resolution. [5][6][7][8] [edit] Wi-Fi Alliance Main article: Wi-Fi Alliance The Wi-Fi Alliance promotes standards in order to improve the interoperability of wireless local area network products based on the IEEE 802.11 standards. It is a consortium of separate and independent companies, and agrees on a set of common interoperable products based on the family of IEEE 802.11 standards.[9] The Wi-Fi Alliance certifies products with a set of defined test procedures. The manufacturers with membership of Wi-Fi Alliance and whose products pass these tests can mark their products and packaging with the Wi-Fi logo.[10] [edit] Naming The term Wi-Fi suggests Wireless Fidelity, compared with the long-established audio recording term High Fidelity or Hi-Fi. Wireless Fidelity has often been used in an informal way, even by the Wi-Fi Alliance itself, but officially the term Wi-Fi does not mean anything. The term Wi-Fi, first used commercially in August 1999,[11] was coined by a brand consulting firm called Interbrand Corporation that had been hired by the Alliance to determine a name that was "a little catchier than 'IEEE 802.11b Direct Sequence'."[12][13][14] Interbrand invented Wi-Fi as a play on words with Hi-Fi, and also created the yin yang- style Wi-Fi logo. The Wi-Fi Alliance initially complicated matters by stating that it "actually stood for" Wireless Fidelity, as with the advertising slogan, "The Standard for Wireless Fidelity,"[13] but later removed the phrase from their marketing. The Wi-Fi Alliance's early White papers still held on their website say: "…a promising market for wireless fidelity (Wi-Fi) network equipment."[15] and "A Short History of WLANs." The yin yang logo indicates that a product had been certified for interoperability.[16] The Alliance has since downplayed the connection to Hi-Fi. Their official position now is that it is merely a brand name that stands for nothing in particular, and they now discourage the use of the term Wireless Fidelity.[citation needed] [edit] Uses
  3. A roof mounted Wi-Fi antenna A Wi-Fi antenna A Wi-Fi enabled device such as a PC, video game console, mobile phone, MP3 player or PDA can connect to the Internet when within range of a wireless network connected to the Internet. The coverage of one or more interconnected access points — called a hotspot — can comprise an area as small as a single room with wireless-opaque walls or as large as many square miles covered by overlapping access points. Wi-Fi technology has served to set up mesh networks, for example, in London.[17] Both architectures can operate in community networks. In addition to restricted use in homes and offices, Wi-Fi can make access publicly available at Wi-Fi hotspots provided either free of charge or to subscribers to various providers. Organizations and businesses such as airports, hotels and restaurants often provide free hotspots to attract or assist clients. Enthusiasts or authorities who wish to provide services or even to promote business in a given area sometimes provide free Wi- Fi access. There are already more than 300 metropolitan-wide Wi-Fi (Muni-Fi) projects in progress.[18] There were 879 Wi-Fi based Wireless Internet service providers in the Czech Republic as of May 2008.[19][20]
  4. Wi-Fi also allows connectivity in peer-to-peer (wireless ad hoc network) mode, which enables devices to connect directly with each other. This connectivity mode can prove useful in consumer electronics and gaming applications. When wireless networking technology first entered the market many problems ensued for consumers who could not rely on products from different vendors working together. The Wi-Fi Alliance began as a community to solve this issue — aiming to address the needs of the end-user and to allow the technology to mature. The Alliance created the branding Wi-Fi CERTIFIED to reassure consumers that products will interoperate with other products displaying the same branding. Many consumer devices use Wi-Fi. Amongst others, personal computers can network to each other and connect to the Internet, mobile computers can connect to the Internet from any Wi-Fi hotspot, and digital cameras can transfer images wirelessly. Routers which incorporate a DSL-modem or a cable-modem and a Wi-Fi access point, often set up in homes and other premises, provide Internet-access and internetworking to all devices connected (wirelessly or by cable) to them. One can also connect Wi-Fi devices in ad hoc mode for client-to-client connections without a router. Wi-Fi also enables places which would traditionally not have network to be connected, for example bathrooms, kitchens and garden sheds. As of 2007 Wi-Fi technology had spread widely within business and industrial sites. In business environments, just like other environments, increasing the number of Wi-Fi access-points provides redundancy, support for fast roaming and increased overall network-capacity by using more channels or by defining smaller cells. Wi-Fi enables wireless voice-applications (VoWLAN or WVOIP). Over the years, Wi-Fi implementations have moved toward "thin" access-points, with more of the network intelligence housed in a centralized network appliance, relegating individual access- points to the role of mere "dumb" radios. Outdoor applications may utilize true mesh topologies. As of 2007 Wi-Fi installations can provide a secure computer networking gateway, firewall, DHCP server, intrusion detection system, and other functions. [edit] Airport Wi-Fi In September of 2003, Pittsburgh International Airport became the first airport to offer free Wi-Fi throughout its terminal. [21] [edit] City wide Wi-Fi Further information: Municipal wireless network Wikibooks has a book on the topic of Nets, Webs and the Information Infrastructure
  5. A municipal wireless antenna in Minneapolis In the early 2000s, many cities around the world announced plans for a city wide Wi-Fi network. This proved to be much more difficult than their promoters initially envisioned with the result that most of these projects were either canceled or placed on indefinite hold. A few were successful, for example in 2005, Sunnyvale, California became the first city in the United States to offer city wide free Wi-Fi.[22] Few of the Municipal Wi-Fi firms have now entered into the field of Smart grid networks.[23][clarification needed] [edit] Campus wide Wi-Fi The first wi-fi network in the world was actually a campus based network. Pittsburgh's Carnegie Mellon University went live with the first-ever wi-fi network in 1994. [24] [edit] Advantages and challenges A keychain size Wi-Fi detector. [edit] Operational advantages Wi-Fi allows local area networks (LANs) to be deployed without wires for client devices, typically reducing the costs of network deployment and expansion. Spaces where cables cannot be run, such as outdoor areas and historical buildings, can host wireless LANs. Wireless network adapters are now built into most laptops. The price of chipsets for Wi- Fi continues to drop, making it an economical networking option included in even more devices. Wi-Fi has become widespread in corporate infrastructures.
  6. Different competitive brands of access points and client network interfaces are inter- operable at a basic level of service. Products designated as "Wi-Fi Certified" by the Wi-Fi Alliance are backwards compatible. Wi-Fi is a global set of standards. Unlike mobile phones, any standard Wi-Fi device will work anywhere in the world. Wi-Fi is widely available in more than 220,000 public hotspots and tens of millions of homes and corporate and university campuses worldwide.[25] The current version of Wi-Fi Protected Access encryption (WPA2) is not easily defeated, provided strong passwords are used. New protocols for Quality of Service (WMM) make Wi-Fi more suitable for latency-sensitive applications (such as voice and video), and power saving mechanisms (WMM Power Save) improve battery operation. [edit] Limitations Spectrum assignments and operational limitations are not consistent worldwide. Most of Europe allows for an additional 2 channels beyond those permitted in the U.S. for the 2.4 GHz band. (1–13 vs. 1–11); Japan has one more on top of that (1–14). Europe, as of 2007, was essentially homogeneous in this respect. A very confusing aspect is the fact that a Wi-Fi signal actually occupies five channels in the 2.4 GHz band resulting in only three non-overlapped channels in the U.S.: 1, 6, 11, and three or four in Europe: 1, 5, 9, 13 can be used if all the equipment on a specific area can be guaranteed not to use 802.11b at all, even as fallback or beacon. Equivalent isotropically radiated power (EIRP) in the EU is limited to 20 dBm (100 mW). [edit] Reach See also: Long-range Wi-Fi Large satellite dish modified for long-range Wi-Fi communications in Venezuela Wi-Fi networks have limited range. A typical wireless router using 802.11b or 802.11g with a stock antenna might have a range of 32 m (120 ft) indoors and 95 m (300 ft) outdoors. The new IEEE 802.11n however, can exceed that range by more than double. [citation needed] Range also varies with frequency band. Wi-Fi in the 2.4 GHz frequency block has slightly better range than Wi-Fi in the 5 GHz frequency block. Outdoor range with improved (directional) antennas can be several kilometres or more with line-of-sight. In general, the maximum amount of power that a Wi-Fi device can transmit is limited by local regulations, such as FCC Part 15[26] in USA.
  7. Wi-Fi performance decreases roughly quadratically[citation needed] as distance increases at constant radiation levels. Due to reach requirements for wireless LAN applications, power consumption is fairly high compared to some other standards. Technologies such as Bluetooth, which are designed to support wireless PAN applications, provide a much shorter propagation range of
  8. wireless access to their LAN. To turn security on requires the user to configure the device, usually via a software graphical user interface (GUI). Wi-Fi networks that are open (unencrypted) can be monitored and used to read and copy data (including personal information) transmitted over the network, unless another security method is used to secure the data, such as a VPN or a secure web page. (See HTTPS/Secure Socket Layer.) [edit] Population Many 2.4 GHz 802.11b and 802.11g access points default to the same channel on initial startup, contributing to congestion on certain channels. To change the channel of operation for an access point requires the user to configure the device. [edit] Channel pollution For more details on this topic, see Electromagnetic interference at 2.4 GHz. Standardization is a process driven by market forces. Interoperability issues between non- Wi-Fi brands or proprietary deviations from the standard can still disrupt connections or lower throughput speeds on all user's devices that are within range, to include the non- Wi-Fi or proprietary product. Moreover, the usage of the ISM band in the 2.45 GHz range is also common to Bluetooth, WPAN-CSS, ZigBee and any new system will take its share. Wi-Fi pollution, or an excessive number of access points in the area, especially on the same or neighboring channel, can prevent access and interfere with the use of other access points by others, caused by overlapping channels in the 802.11g/b spectrum, as well as with decreased signal-to-noise ratio (SNR) between access points. This can be a problem in high-density areas, such as large apartment complexes or office buildings with many Wi-Fi access points. Additionally, other devices use the 2.4 GHz band: microwave ovens, security cameras, ZigBee devices, Bluetooth devices and (in some countries) Amateur radio, video senders, cordless phones and baby monitors, all of which can cause significant additional interference. General guidance to those who suffer these forms of interference or network crowding is to migrate to a Wi-Fi 5 GHz product, (802.11a, or the newer 802.11n if it has 5 GHz support) because the 5 GHz band is relatively unused, and there are many more channels available. This also requires users to set up the 5 GHz band to be the preferred network in the client and to configure each network band to a different name (SSID). It is also an issue when municipalities,[28] or other large entities such as universities, seek to provide large area coverage. This openness is also important to the success and widespread use of 2.4 GHz Wi-Fi. [edit] Hardware [edit] Standard devices
  9. An embedded RouterBoard 112 with U.FL-RSMA pigtail and R52 mini PCI Wi-Fi card widely used by wireless Internet service providers (WISPs) in the Czech Republic. OSBRiDGE 3GN - 802.11n Access Point and UMTS/GSM Gateway in one device. USB wireless adapter A wireless access point (WAP) connects a group of wireless devices to an adjacent wired LAN. An access point is similar to a network hub, relaying data between connected wireless devices in addition to a (usually) single connected wired device, most often an ethernet hub or switch, allowing wireless devices to communicate with other wired devices. Wireless adapters allow devices to connect to a wireless network. These adapters connect to devices using various external or internal interconnects such as PCI, miniPCI, USB, ExpressCard, Cardbus and PC Card. Most newer laptop computers are equipped with internal adapters. Internal cards are generally more difficult to install. Wireless routers integrate a Wireless Access Point, ethernet switch, and internal Router firmware application that provides IP Routing, NAT, and DNS forwarding through an integrated WAN interface. A wireless router allows wired and wireless ethernet LAN devices to connect to a (usually) single WAN device such as cable modem or DSL modem. A wireless router allows all three devices (mainly the access point and router) to be configured through one central utility. This utility is most usually an integrated web server which serves web pages to wired and wireless LAN clients and often optionally to
  10. WAN clients. This utility may also be an application that is run on a desktop computer such as Apple's AirPort. Wireless network bridges connect a wired network to a wireless network. This is different from an access point in the sense that an access point connects wireless devices to a wired network at the data-link layer. Two wireless bridges may be used to connect two wired networks over a wireless link, useful in situations where a wired connection may be unavailable, such as between two separate homes. Wireless range extenders or wireless repeaters can extend the range of an existing wireless network. Range extenders can be strategically placed to elongate a signal area or allow for the signal area to reach around barriers such as those created in L-shaped corridors. Wireless devices connected through repeaters will suffer from an increased latency for each hop. Additionally, a wireless device connected to any of the repeaters in the chain will have a throughput that is limited by the weakest link between the two nodes in the chain from which the connection originates to where the connection ends. [edit] Distance records Electronics portal Distance records (using non-standard devices) include 382 km (237 mi) in June 2007, held by Ermanno Pietrosemoli and EsLaRed of Venezuela, transferring about 3 MB of data between mountain tops of El Aguila and Platillon.[29][30] The Swedish Space Agency transferred data 310 km (193 mi), using 6 watt amplifiers to reach an overhead stratospheric balloon.[31] [edit] Embedded systems Embedded serial-to-Wi-Fi module Wi-Fi availability in the home is on the increase.[32] This extension of the Internet into the home space will increasingly be used for remote monitoring.[citation needed] Examples of remote monitoring include security systems and tele-medicine. In all these kinds of implementation, if the Wi-Fi provision is provided using a system running one of
  11. operating systems mentioned above, then it becomes unfeasible due to weight, power consumption and cost issues. Increasingly in the last few years (particularly as of early 2007), embedded Wi-Fi modules have become available which come with a real-time operating system and provide a simple means of wireless enabling any device which has and communicates via a serial port.[33] This allows simple monitoring devices – for example, a portable ECG monitor hooked up to a patient in their home – to be created. This Wi-Fi enabled device effectively becomes part of the internet cloud and can communicate with any other node on the internet. The data collected can hop via the home's Wi-Fi access point to anywhere on the internet. [34] These Wi-Fi modules are designed so that designers need minimal Wi-Fi knowledge to wireless-enable their products. [edit] Network security The main issue with wireless network security is its simplified access to the network compared to traditional wired networks such as ethernet. With wired networking it is necessary to either gain access to a building, physically connecting into the internal network, or break through an external firewall. Most business networks protect sensitive data and systems by attempting to disallow external access. Thus being able to get wireless reception provides an attack vector, if encryption is not used or can be defeated. [35] Attackers who have gained access to a Wi-Fi network can use DNS spoofing attacks very effectively against any other user of the network, because they can see the DNS requests made, and often respond with a spoofed answer before the queried DNS server has a chance to reply.[36] [edit] Securing methods A common but unproductive measure to deter unauthorized users is to suppress the AP's SSID broadcast, "hiding" it. This is ineffective as a security method because the SSID is broadcast in the clear in response to a client SSID query. Another unproductive method is to only allow computers with known MAC addresses to join the network. The fault with this method is MAC addresses can often, but not always, be set by a user (spoofed) with minimal effort. If the eavesdropper has the ability to change his MAC address, then he may join the network by spoofing an authorized address. Wired Equivalent Privacy (WEP) encryption was designed to protect against casual snooping, but is now considered completely broken. Tools such as AirSnort or Aircrack- ng can quickly recover WEP encryption keys. Once it has seen 5-10 million encrypted packets, AirSnort can determine the encryption password in under a second;[37] newer tools such as aircrack-ptw can use Klein's attack to crack a WEP key with a 50% success rate using only 40,000 packets.
  12. To counteract this in 2002, the Wi-Fi Alliance blessed Wi-Fi Protected Access (WPA) which uses TKIP as a stopgap solution for legacy equipment. Though more secure than WEP, it has outlived its designed lifetime, has known attack vectors and is no longer recommended. In 2004, the full IEEE 802.11i (WPA2) encryption standards were released. If used with a 802.1X server or in pre-shared key mode with a strong and uncommon passphrase WPA2 is still considered secure, as of 2009. [edit] Piggybacking Main article: Piggybacking (internet access) During the early popular adoption of 802.11, providing open access points for anyone within range to use was encouraged to cultivate wireless community networks;[38] particularly since people on average use only a fraction of their downstream bandwidth at any given time. Recreational logging and mapping of other people's access points has become known as wardriving. It is also common for people to use open (unencrypted) Wi-Fi networks as a free service, termed piggybacking. Indeed, many access points are intentionally installed without security turned on so that they can be used as a free service. Providing access to one's Internet connection in this fashion may be contrary to the Terms of Service or contract with the ISP. These activities do not result in sanctions in most jurisdictions, however legislation and case law differ considerably across the world. A proposal to leave graffiti describing available services was called warchalking. In a Florida court case, owner laziness was determined not to be a valid excuse. [39] Piggybacking is often unintentional. Most access points are configured without encryption by default, and operating systems such as Windows XP SP2, Mac OS X or Ubuntu Linux may be configured to automatically connect to any available wireless network. A user who happens to start up a laptop in the vicinity of an access point may find the computer has joined the network without any visible indication. Moreover, a user intending to join one network may instead end up on another one if the latter's signal is stronger. In combination with automatic discovery of other network resources (see DHCP and Zeroconf) this could possibly lead wireless users to send sensitive data to the wrong middle man when seeking a destination (see Man-in-the-middle attack). For example, a user could inadvertently use an insecure network to login to a website, thereby making the login credentials available to anyone listening, if the website is using an insecure protocol like HTTP.
Đồng bộ tài khoản